The present invention relates to a method and apparatus for synchronously detecting a magnetocardiogram. The present invention also relates to methods and apparatus for synchronously adding magnetocardiograms. More particularly, the present invention relates to a method and an apparatus for detecting a magnetocardiogram in synchronism with a Q-R-S group of an electrocardiogram, the magnetocardiogram being measured by a superconducting quantum interference device magnetic flux meter including a superconducting quantum interference device (hereinafter referred to as a SQUID). The present invention also relates to methods and apparatus for adding magnetocardiograms corresponding to individual cycles of a magnetocardiogram in synchronism with a Q-R-S group of an electrocardiogram, the magnetocardiogram being measured by a SQUID magnetic flux meter including a SQUID.
It is known that a SQUID is capable of detecting magnetic flux with extremely high sensitivity. Wi&h attention to this characteristic, a SQUID is applied to various apparatus which are used in various technical fields. When an object of magnet flux measurement is a living organism, it is strongly desired that magnetic flux is measured without an invasive procedure. Therefore, it is proposed that a magnetocardiogram is measured using a SQUID magnetic flux meter.
Conventionally a method for measuring an electrocardiogram is generally employed so as to diagnose diseases of the heart. For example, the method is insufficient for estimating a position of a part of the heart which is to be singled out during an operation, that is, satisfactory estimation of a position is not obtained. The reason for insufficient estimation is that accurate wave forms of electrocardiograms cannot be obtained and only almost uniform wave forms of electrocardiograms ere accordingly obtained at all measurement times because an electrocardiogram is an indirect measurement method, and an electrocardiogram depends on relative positions, sizes, electric conductivities and the like of the anatomy between the heart and the surface of the body, and other internal organs, may vary a fair amount according to the person to be measured. To overcome the disadvantage mentioned above, a method for directly pricking or contacting a needle electrode or meshed electrode to the heart is employed so as to estimate an accurate position in singling out a portion of the heart. Disadvantages arise in that a time period for opening a patient's thorax for an abdominal operation is lengthened and a required time period for an operation on the heart is also lengthened. Therefore, measurement without invasion for estimating an accurate position in a short period of time for singling out a portion of the heart, is strongly demanded.
A dc-SQUID is being widely used because two Josephson junctions having similar characteristics can be obtained due to improvements in thin film manufacturing engineering in recent years. Measuring a magnetocardiogram using a SQUID magnetic flux meter is carried out for trial by taking the demand into consideration. When magnetocardiograms are to be measured, the measured magnetocardiograms differ from one another due to individuals and measurement situations. Seizing a magnetic field corresponding to a situation of heart at a desired time is accordingly difficult. It is proposed that a magnetic field is easily seized by determining a trigger level for triggering the magnetocardiogram with an R-wave of an electrocardiogram by using the electrocardiogram as a triggering signal and by detecting the magnetocardiogram in synchronism with the electrocardiogram. It is also proposed that magnetocardiograms corresponding to individual cycles of a magnetocardiogram are added and averaged in synchronism with an R-wave of an electrocardiogram so as to improve a signal to noise ratio (hereinafter referred to as S/N ratio).
The electrocardiogram includes characteristic variations due to living organisms, as is mentioned above, and a measured electrocardiogram includes a characteristic fluctuation (1/t) refer to FIG. 8, for example). That is, the electrocardiogram may include not only variations in proper shapes and amplitude of Q-R-S groups but also an offset which is convoluted to a ground level due to myoelectric potential. Therefore, when a trigger level is simply determined which is shown by a dashed line in FIG. 9(A), disadvantages arise in that a magnetocardiogram may be triggered with T-waves of an electrocardiogram and that shifting in synchronization may occur by several to several tens of milliseconds as is shown in FIG. 9(A). Consequently, magnetocardiograms corresponding to individual cycles of a magnetocardiogram cannot be added in synchronism accurately, thereby the improvement of the S/N ratio to a predetermined value cannot be performed. Specifically, a level of an R-wave is lowered while levels of a P-wave and T wave are relatively raised by performing the synchronous addition of magnetocardiograms as is shown in FIG. 9(C).
It may be proposed that a peak position of the Q-R-S group is to be detected. Then, similar disadvantages as in above-mentioned arise because a shape of the R-wave greatly varies in the neighbouring portion to the peak Of the R-wave with respect to the zero-crossing center of the Q-R-S group as is shown in FIG. 9(B).
Furthermore, a disadvantage arises in that determination of the triggering level is difficult because an amplitude of the T wave may be great due to a guiding method of an electrocardiogram or differences between persons.